Broad band circuits



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N. T. LAVOO BOARD BAND CIRCUITS Filed Nov. 8,

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LENGTH FREQUENCY /A/ GC May 30, 1967 .e PM ls Attorney.

/N INCHES (h) United States Patent O 3,323,072 BROAD BAND CIRCUITS Norman T. Lavoo, Latham, N.Y., assigner to General Electric Company, a corporation of New York Filed Nov. 8, 1963, Ser. No. 322,317 6 Claims. (Cl. 330-56) ABSTRACT F THE DISCLOSURE A quarter-wave length shielded two wire cavity resonator circuit for obtaining a broad band response in a space charge control tube includes a circular shield having two spaced, parallel, nonconcentric conductors extending longitudinally of the shield. One end of the shield is closed while the other has an opening through which the tube passes so that its anode plugs into one of the conductors. The structure provides a double tuned output circuit for the tube, operating in two modes; a mode in which the shield provides a return path for current in the two conductors; and a or standard, transmission line mode consisting of the two conductors.

This invention relates to broad band circuits and particularly to broad band coupling circuits for vacuum tubes operated at high frequencies.

The advancing state of microwave technology makes desirable more and more bandwidth with high gain in microwave transmission systems and the like. Not only are wide bandwidths with high gain desirable, but minimum phase shift is also necessary. Miniature microwave triodes and tetrodes fulll these requirements in amplifying tubes, particularly the minimum phase shift consideration, and have the additional advantage of high density cathode emission making possible relatively high power operation. However, the bandwidth and gain of the transmission system is usually limited by the bandwidth of the input and output devices which complete the vacuum tube circuit. It is a purpose of the present invention to provide a complete vacuum tube circuit of the above type having greater bandwidth than heretofore available.

In accordance with an embodiment of the present invention, a vacuum tube amplier is employed in conjunction with a shielded two conductor line output cavity including a two conductor line with a circumferential shield thereabout and closed ends. The vacuum tube output is coupled to one of the two conductors forming the line While the output of the circuit is taken from the other conductor. This construction leads to two cavity operating modes, one being a substantially coaxial mode where current ow in yboth the central conductors is in the same direction, the current returning in the surrounding shield, and the other mode being a shielded two wire line mode. These two modes resonate at spaced frequencies providing a pass band on the order of 13 percent at one gigacycle and gains of 12 db with presently commercially available triodes.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. l is a vertical cross-sectional view of a broad band circuit in accordance with the present invention particularly illustrating the output cavity and a first input circuit,

ice

FIG. la is a cross-section of the output cavity in accordance with the present invention taken at 1a-1a in FIG. 1,

FIG. lb is a cross-section of an input circuit in accordance with the present invention taken at 1li-1b in FIG. l,

FIG. 2 is another View of a broad band amplifier in accordance with the present invention particularly illustrating a second input circuit,

FIG. 3 is a gain versus frequency characteristic provided in accordance with the present invention,

FIG. 4 is the plot of frequency versus separation of the inner conductors for the FIG. 1 output cavity, and

FIG. 5 is a plot of frequency versus the diameter of inner conductors for the output cavity of FIG. 1.

Referring to FIGS. l and la, a miniature disk seal vacuum tube 1 having ring shaped terminals is provided with an output cavity 2 having a physical length of somewhat less than one-fourth wavelength, at a frequency on the order of one gigacycle, as measured between the end walls 3 and 4 of the cavity. The tube is also provided with an input circuit 5. The miniature tube is of the general type described and claimed in Boggs Patent 2,680,824, assigned to the assignee of the present invention. Specifically a General Electric type 7768 tube was employed. The tube includes an anode terminal 6, a grid ring 7 and a cathode ring or terminal 8, as well as filament connections 9 and 10. The tube is conveniently supported at grid ring 7 joined to cavity end wall 4 for grounded grid operation. i

The output cavity in accordance with the present invention is a shielded two conductor line cavity including line conductors 11 and 12 with a cylindrical shield 13 thereabout. The two line conductors are centrally located lin the cavity and spaced from the surrounding shield in the manner of a two wire shielded line. In the specific construction of the illustrated embodiment, the line conductors were located Vs of an inch on either side of cavitys center line, while the inside diameter of the cavity was 1% inches.

The line conductors 11 and 12 are both effectively grounded for radio frequencies at end wall 3 of the cavity gap. End wall 3 is the inner surface of a first conductive plate 14 to which line conduc-tor 12 `is attached and through which conductor 12 extends. Plate 14 closes the cavity, being adjustably supported at its periphery by circumferential shield 13. Plate 14 may be moved vertically for adjusting the cavity height. A second conductive plate 15 adjoins plate 14 and is supported thereby with only mica insulation 16 separating the two providing capacitive coupling therebetween. Plate 15, which is spaced from the circumferential shield, supports line conductor 11, the latter extending through an insulated aperture 17 in plate 14. Line conductor 12 also extends through an insulated aperture 18 in plate 15.

The foregoing end wall construction couples both line conductors 11 and 12 to the circumferential shield at R.F. frequencies but insulates them one from the other. Only conductor 12 is physically joined to the shield 13. Tube 1 is positioned with its anode terminal 6 in line with conductor 11 and conductor 11 is attached thereto. Then B-ilead 19 is connected to the remote end of conductor 11. For this reason, plate 15 is insulated from the shield except at the operating frequency and the plates 14 and 15 form a bypass to ground. The described construction is also of particular advantage in that conductor 11 and place 15 provide considerable heat dissipation from the tube anode. The cavity, being less than one-fourth wavelength high, is relatively short, and conductor 11 has a relatively large diameter (for a given Zo), thereby achieving excellent conduction of heat away from anode terminal 6.

The remaining conductor 12 is supported between plate 14 and insulated fastener 20 extending through cavity wall 4. Fastener 20 also secures metal plate 21 in contacting relationship with cavity wall 4. The lower end of conductor 12 is provided with a flat faced extension 22 which is conveniently cubical in shape. This extension presents one face in parallel juxtaposition with the plate 21 and is separated therefrom by means of insulating material 23. The capacitance therebetween provides end-loading to balance the grid-anode capacitance of tube 1. The electrical length of cavity 2 will be one-quarter wavelength as foreshortened by these capacitances.

A perpendicular face of extension 22 is disposed in parallel relation to a second plate 24 supported by coaxial connector 25 extending through the near wall of circumferential shield 13 and the outer connector of which contacts the shield 13. This connector then forms the output line for the Output cavity 2. Insulating material 26 separates extension 22 from plate 24. Plate 24 is conveniently adjustable in its distance from extension 22 whereby the capacitive coupling therebetween may be a1- tered for the purpose of matching the output cavity to a load.

A number of methods are possible to alter the frequency separation of the two resonant modes that result because of the particular geometry chosen. Such methods depend on the different field distribution in the two modes. For instance, the -i--lmode is preferentially perturbed by means of metal or dielectric obstacles around the outer periphery of the circuit. On the other hand, the -lor two wire line type mode offers the possibility of preferentially perturbing primarily this mode by means of an obstacle between the two conductors. As an example of the latter, a flat movable vane 27 is supported from a rod 28 extending through and insulated from plate 15. Rod 28 is conveniently turnable so that vane 27 is disposed with its fiat sides facing the conductors 11 and 12,

`or perpendicular thereto, or at some direction in between.

This vane may be used for fine tuning of the frequency modes of output cavity 2.

Input circuit extends in a direction perpendicularly away from tube 1 and substantially coaxial therewith on the side of tube 1 opposite cavity 2. One purpose of the input circuit is to balance the grid-cathode capacitance of the tube. Input circuit 5 is approximately one-half electrical wavelength between grid ring 7 and input line 50, the first one-quarter wave of which is effectively shorrrned by the grid capacitance of the tube. Filament connections 9 and 10 are made at the location of a null point found to exist along the input circuit at the end of the first quarter wave, and these connections are brought outside the circuit through feedthrough bypass capacitors 51 and 52 for coupling to an appropriate filament supply source.

The remaining one-quarter wavelength of the input circuit is a coaxial matching section having an inner conductor 53 and an outer conductor54, the latter being joined to the grid ring and the underside of the cavity 2. Bypass capacitors 51 and 52 extend through outer conductor 54. Central conductor 53 is provided with fingers 55 at tube 1 for contacting cathode connection 8. The conductor is apertured to pass feedthrough capacitor 52. The remaining filament connection 9 joins the central conductor.

The quarter-wave matching section between the null point and line 50, is adjustable in its impedance. Central conductor 53 is formed of a plurality` of longitudinally separated segments including stationary segments 56 and 57 and movable segments 58 and 59, more fully illustrated in the cross-section of FIG. 1b. The stationary segments are rigidly joined at either end to end plates 60 and 61. A central support beam 62, extending from one end plate to the other, acts to separate and position the stationary segments. Support beam 62 also centrally supports a reverse threaded member 63 whose threads are matingly received by threaded apertures in the movable Segments 58 and 59. The threaded member 63 has reverse threads on either side of beam 62, and a slot 64 at one end thereof for adjustable positioning of the movable segments through an aperture 65 in outer conductor 54. When member 63 is turned in one direction it acts to draw the movable segments closer together, and when turned in the other direction it acts to move them further apart, in the direction of outer conductor 54. This construction provides adjustable impedance matching between a null point near the filament connections and the 50 ohm input line 5t).

Flexible conductive bands 66 secure the movable segments to the end plates, while a central extension 67 connects end plate 61 with the central conductor of input line 5f). The outer conductor of input line 50 is, of course, joined to outer conductor 54.

An alternative input circuit is illustrated in FIG. 2. FIG. 2 shows a chassis 40 on the reverse side of which is mounted wall 4 of output cavity 2, with tube 1 protruding through chassis 40 in the foreground. Grid terminal ring 7 is shown in conductive connection with chassis 40 and cathode terminal 8 is joined to fiat conductor 41 forming one side of a strip line with chassis 40. R.F. chokes 42 and 43 join filament connections 9 and 10 to feedthrough bypass insulators 44 and 45 extending through the wall of chassis 40 for connection to an appropriate source of filament power. A third R.F. choke 46 joins filament 10 to a voltage null point 47 on flat conductor 41.

The strip line together with vacuum tube 1 form a foreshortened one-quarter wavelength between grid terminal ring 7 and voltage null point 47. This length is reduced physically because of the foreshortening eifect of the grid cathode capacity of the tube.

The remainder of flat conductor 41 extends approximately one-quarter wavelength from null point 47 to the center conductor 48 of an input line, the outer conductor of the input line being joined to chassis 40. A block of high dielectric constant ceramic material 49 is disposed between flat conductor 41 and chassis 40. A material known commercially as Stycast-HiK ceramic is convenient, having a dielectric constant of 9. The high dielectric material not only tends to foreshorten the line, but also changes the strip line impedance to provide impedance matching between null point 47 and the coaxial input.

Both the input circuits described are appropriate for matching a 50 ohm input to the stage over a wide range of frequencies. The output circuit is more important in determining the bandwidth of the stage. The tube output impedance is higher, but is likewise to be matched to coaxial output connector 25 in FIG. l similarly having an impedance on the order of 50 ohms. The output cavity 2 is therefore resonant and, in particular, is doubly resonant at spaced frequencies to define a broad pass band.

The output cavity operates in two different modes. The first is a mode characteristic of a two conductor line, and in this case, a shielded two conductor line. In this mode, as in each mode, both conductors 11 and 12 are grounded for radio frequencies at cavity wall 3. However, at the lower end of the cavity, approximately one-quarter wavelength away, the voltage on these conductors is approximately out of phase. We shall designate this as the mode.

Having a shield around the two lines makes it possible for the shield to provide the return path for current in a second or coaxial mode. In this case the lower ends of conductors 11 and 12 are in phase and we shall designate this as the -i--lmode.

The cavity resonances are similar to a parallel resonant circuit having capacitors in series with an inductance for the mode, and having the same capacitors in parallel with an inductance for the +-imode. In the illustrated embodiment I provide a symmetrical circuit wherein the tube grid-anode capacitance terminating line 11 may be equal to the capacitance terminating line 12, this being the capacitance between plate 21 and extension 22 of conductor 12 (plus that introduced by the coupling to the external load). The equations defining the resonant frequencies for the two modes are as follows:

In these equations:

These equations may be used to calculate the separation of the modes. For instance, in the special case where the shield diameter is 1% inches and the two-wire line consists of 1A; inch diameter conductors, the effect of variation of distance between the two conductors is shown in FIG. 4. The capacitances terminating the lines were taken to be 2.5 picofarads and the upper mode has been adjusted in each to be 100() mc./s. by means of adjusting the length of the resonator 2 between walls 3 and 4 by positioning plate 4. Note that the -i--lmode may easily be made to 20 percent less in frequency than the mode. FIG. 5 illustrates a similar situation except in this case the spacing between conductors has been fixed at 5%; inch and the diameter of the conductors is varied. This variable has considerably less effect on the mode separation than in the previous figure.

The output cavity circuit in accordance with the present invention contributes several important advantages. A very high cavity characteristic impedance is found to 4be obtainable in a compact structure. This high characteristic impedance contributes to a favorable impedancebandwidth product. Additionally the shortness of the resonator makes the task of conducting thermal energy away from the anode terminal easier, or, stated somewhat differently, a large diameter anode line may be used without seriously irnpairing the impedance-bandwidth factor. The bandwidth characteristic is plotted in FIG. 3. Note the gain attained in the illustrated embodiment is 12 db with a 13 percent bandwidth measured to the 1 db point. The peaks of the characteristic are, of course, representative of the -1--iand resonances. The saddle or dip in the top of the characteristic is adjusted by varying the loading achieved with the capacitor formed between extension 22 and plate 24 attached to the output connector. As illustrated in the FIG. 3 characteristic, this dip is adjusted to approximately 1/2 db.

While I have shown and described particular embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A broad band circuit :comprising a shielded two conductor line cavity resonator including a two conductor line with a cylindrical shield thereabout and closed ends, means connecting the two conductors of said line to one end at radio frequencies, a Vacuum tube including an anode terminal, a `grid terminal and a cathode terminal having its grid terminal coupled to the remaining closed end of said resonator and having its anode terminal extending into said resonator and coupled to one of said conductors, an output line, and means coupling said output line to the remaining conductor, an input circuit comprising a coaxial line extending from said tube the center conductor of which is coupled to said cathode terminal and the outer conductor of which is coupled to said grid terminal, said coaxial line havin-g an electrical length of one-half wavelength of the operating frequency, an input line coupled to the remaining end of said coaxial line, the center conductor of said coaxial line being formed of a plurality of longitudinally separated segments including stationary segments and movable segments provided with threaded apertures, and translation means for moving said movable segments relative to the outer conductor of said coaxial line, said translation means comprising threaded members supported by stationary segments of said inner conductor and matingly received by said threaded apertures of movable segments of said inner conductor for adjustably supporting said movable segments of said inner conductor.

2. A broad lband circuit comprising a cavity resonator having two operating modes comprising a cylindrical shield, two spaced, parallel, and non-concentric conductors extending longitudinally of said shield, conductive means closing one end of said shield and connected to said conductors for alternating currents, said conductive means being insulated from one of said conductors for unidirectional currents, a conductive plate closing the other end of said shield, said plate having an eccentric aperture therein, an electron discharge device extending into said resonator through said aperture, said device having an anode insulated from said plate and connected to said one conductor, the other of said conductors having one of its ends capacitively coupled to said plate and being connected to said conductive means for unidirectional currents, whereby said two conductors operate in a rst mode as a transmission line in which current flows down one conductor and returns over the other, and said two conductors and said shield operate in a second mode in which current flows down the two conductors and returns over said shield.

3. The circuit of claim 2 in which said device is of the disk seal type and has an external grid ring which is conductively connected to said conductive plate.

4. The circuit of claim 2 in which output means are coupled to said other conductor adjacent its said one end.

5. The circuit of claim 2 in which adjustable tuning means are provided for said resonator comprising a movable vane positioned between said conductors and supported by said conductive means.

6. The circuit of claim 3 which includes means to vary the capacitive coupling of said other conductor to balance the -grid-anode capacitance of said device.

References Cited UNITED STATES PATENTS 2,681,427 6/1954 Brown et al. 333-82 X 2,847,518 8/1958 Lavoo et al. 330-56 2,948,858 8/ 1960 Sta-meson 330-56 3,098,206 7/1963 Moulton 330-56 X 3,113,278 12/1963 Okwit 333-82 X 3,138,765 6/1964 Bridges et al. 330-56 X 3,153,765 10/1964 Weaver 330-56 ROY LAKE, Primary Examiner.

N. KAUF MAN, Assistant Examiner. 

1. A BROAD BAND CIRCUIT COMPRISING A SHIELDED TWO CONDUCTOR LINE CAVITY RESONATOR INCLUDING A TWO CONDUCTOR LINE WITH A CYLINDRICAL SHIELD THEREABOUT AND CLOSED ENDS, MEANS CONNECTING THE TWO CONDUCTORS OF SAID LINE TO ONE END AT RADIO FREQUENCIES, A VACUUM TUBE INCLUDING AN ANODE TERMINAL, A GRID TERMINAL AND A CATHODE TERMINAL HAVING ITS GRID TERMINAL COUPLED TO THE REMAINING CLOSED END OF SAID RESONATOR AND HAVING ITS ANODE TERMINAL EXTENDING INTO SAID RESONATOR AND COUPLED TO ONE OF SAID CONDUCTORS, AN OUTPUT LINE, AND MEANS COUPLING SAID OUTPUT LINE TO THE REMAINING CONDUCTOR, AN INPUT CIRCUIT COMPRISING A COAXIAL LINE EXTENDING FROM SAID TUBE THE CENTER CONDUCTOR OF WHICH IS COUPLED TO SAID CATHODE TERMINAL AND THE OUTER CONDUCTOR OF WHICH IS COUPLED TO SAID GRID TERMINAL, SAID COAXIAL LINE HAVING AN ELECTRICAL LENGTH OF ONE-HALF WAVELENGTH OF THE OPERATING FREQUENCY, AN INPUT LINE COUPLED TO THE REMAINING END OF 